Influence of linkers on the Kuratowski-type secondary building unit in nickel single-site MOFs for ethylene oligomerization catalysis: a computational study

Abstract

The use of metal–organic frameworks (MOFs) as heterogeneous catalysts has been considered an efficient alternative for petrochemical and industrial applications. Ni-MFU-4l (dibenzodioxin-type backbone) and Ni-CFA-1 (biphenyl-type backbone) are heterogeneous nickel catalysts based on MOFs, which were designed based on grafting well-known homogeneous scorpionate catalysts or the Kuratowski-type secondary building unit behaving as a platform catalytically active for ethylene oligomerization catalysis. From the experimental results of the Dincă group, Ni-CFA-1 is a more economical alternative than Ni-MFU-4l with similar catalytic performance. However, understanding the origin of organic linker effects on the catalytic activity and selectivity is still limited. Here we report the first comprehensive computational study of ethylene oligomerization catalysis on the Kuratowski-type secondary building units of nickel single-site MOFs to rationalize and predict the influence of potential factors in the coordination environment on the catalytic performance towards ethylene. We found for both catalysts through DFT calculations that the rate-determining step is the first ethylene uptake, Ni-MFU-4l provides a slight rate increase, which is reflected by the higher experimental TOF observed for this catalyst. Although both catalysts can promote the chain propagation via a second ethylene uptake at higher temperatures (50 °C), this reaction is least likely to occur at room temperature. The preferred reaction mechanism involves an earlier β-hydrogen elimination reaction leading to 1-butene as the main product. β-Hydrogen elimination reactions catalyzed by Ni-MFU-4l were found to be slower due to the increased stability of the intermediate species in the catalytic cycles, whereas the use of the sterically demanding Ni-CFA-1 derivative achieved higher activity for this process. Energy decomposition analysis revealed the important role of the linkers in these complexes. When the Ni-CFA-1 derivative catalyzes the reaction, the steric hindrance close to the nickel center, and the greater donor ability of the biphenyl linker, decrease the activity. It must be finely tuned because this kind of linker may favor β-hydrogen elimination reactions leading to undesired termination and isomerization reactions. We expect the mechanistic insights revealed in this study on the Ni-single site Kuratowski-type secondary building unit's selectivity for ethylene oligomerization catalysis could offer unique insights to guide future experimental organic linker design strategies.